The most common method used to interface high-level analog field transmitters to a process control system is the 4-20 mA current loop. A 4-20 mA current loop can be implemented as a point-to-point or multi-drop circuit mainly used in the process automation field to transmit signals from instruments and sensors in the field to a controller. It sends an analog signal from 4 to 20 mA that represents 0 to 100% of some process variable. As a current loop signal, 4-20 mA also powers the sensor transmitter on the same wire pair, and 4-20 mA provides increased immunity against interference than a voltage-based line.
FIG. 1 illustrates a schematic diagram of a prior art system 100 that includes a 4-20 mA current loop 101. The configuration of system 100 depicted in FIG. 1 represents a common technique for transmitting sensor information via a sensor 106 in many industrial process-monitoring applications, including the use of a sensor to measure physical parameters such as temperature, pressure, speed, liquid flow rates, etc. Transmitting sensor information via the current loop 101 is particularly useful when the information has to be sent to a remote location over long distances (e.g., 1000 feet, or more).
System 100 generally includes sensor 106 in association with a transmitter 102, a power supply 108, and a process monitor/controller component 104 that includes an anti-aliasing filter 118 that is connected to and provides a signal to an Analog-to-Digital Converter (ADC) 120, which in turn provides a signal to a control system or control device 121. The transmitter 102 typically includes electronic circuitry 110, in communication with sensor 106. The operation of system 100 and its loop 101 is fairly straightforward: the output signal from sensor 106 is first converted to a corresponding current via the electronic circuitry 110, with, for example, 4 mA normally representing the zero-level output of sensor 106, and 20 mA, for example, representing the full-scale output of sensor 106. Then, a receiver at the remote end converts the 4-20 mA current into a voltage which in turn can be further processed by the process monitor/controller component 104.
Transmitting an output of sensor 106 as a voltage over long distance, however, has several drawbacks. Unless very high input-impedance devices are used, transmitting voltages over long distances produces correspondingly lower voltages at the receiving end due to wiring and interconnect resistances. High-impedance instruments, however, can be sensitive to noise pickup since the lengthy signal-carrying wires often run in close proximity to other electrically noisy system wiring. Shielded wires can be used to minimize noise pickup, but their high cost may be prohibitive when long distances are involved. Sending a current over long distances produces voltage losses proportional to the wiring's length. However, these voltage losses—also known as “loop drops”—do not reduce the 4-20 mA current as long as the transmitter and loop supply can compensate for these drops. The magnitude of the current in the loop 101 is not affected by voltage drops in the wiring of system 100 since all of the current (i.e., electrons) originating at the negative (−) terminal of the loop power supply has to return back to its positive (+) terminal.
In a configuration such as system 100 and loop 101, the transmitter varies the current to communicate the process variable. At the receiving end, the current is converted to a voltage by passing it through a precision resistor. This voltage is filtered to remove noise and prevent frequency aliasing, then periodically converted (at a frequency called the “scan rate” or “sampling rate”) to a digital value and passed to the control system of control component 104.
If the field wiring opens, the current will drop to zero virtually instantly. The output of the anti-aliasing filter, however, drops at a rate determined by its time constant. This can result in control action on some number of bad input values before the control system can detect the open-wire condition.
It is believed that a solution to these problems can be achieved through the design and implementation of a new and novel technique for detecting an open wire detection by sensing the input signal with two different frequency responses at two or more different locations within the input circuit. Such a technique, including a method and system thereof, is described in greater detail herein.